1. Field of the Invention
This invention relates generally to materials for spark plug electrodes, and particularly to materials of the electrodes.
2. Description of the Prior Art
Spark plugs are widely used to initiate combustion in an internal combustion engine. Spark plugs typically include a ceramic insulator, a conductive shell surrounding the ceramic insulator, a center electrode disposed in the ceramic insulator, and a ground electrode operatively attached to the conductive shell. The electrodes each have a sparking end located proximate one another and defining a spark gap therebetween. Such spark plugs ignite gases in an engine cylinder by emitting an electrical spark jumping the spark gap between the center electrode and ground electrode, the ignition of which creates a power stroke in the engine. Due to the nature of internal combustion engines, spark plugs operate in an extreme environment of high temperature and various corrosive combustion gases and therefore should be fabricated of appropriate materials. When the electrodes are not fabricated of appropriate materials, the extreme working conditions may gradually increase the width of the spark gap between the center electrode and ground electrode, and may induce the misfire of spark plugs and cause subsequent loss of engine power and performance.
Spark plug electrodes often include a core formed of copper (Cu) and a clad formed of a nickel (Ni) alloy due to the high temperature performance of Cu and Ni. Ni alloys are resistant to erosion and corrosion, and Cu provides a high thermal conductivity and thus a controlled operating temperature of the electrode. An example of an existing electrode includes a core formed of 100 wt % Cu and a clad formed of a Ni alloy including 14.5-15.5 wt % Cr, 7.0-8.0 wt % Fe, 0.2-0.5 wt % Mn, and 0.2-0.5 wt % Si and a balance of Ni.
The existing electrodes including a Cu core and Ni alloy clad experience large temperature gradients when the engine runs between full throttle and idle operation. There is a significant difference in thermal expansion of the Cu core and the Ni clad, which causes undesirable swelling and thermal mechanical stresses. The swelling may increase the width of the spark gap unexpectedly. At high temperatures, such as greater than 500° C., compressive axial thermal stress builds up in the Cu core due to the higher thermal expansion coefficient of Cu than that of Ni. The Cu can undergo a time dependent creep deformation under the compressive axial stress. The Cu core shrinks axially and expands radially, which compresses the Ni clad. The Ni clad has a tension stress along the azimuthal direction which may cause cracking in the Ni clad and insulator. FIGS. 5 and 6 show deformation of the electrode and cracks due to thermal stress and creep, which may hinder the performance of the spark plug.